The present invention relates in general to the field of turbocompressors, and in particular to a new and useful method of controlling the surge limit of such turbocompressors.
Such a method is known from "Nachrichten fur den Maschinenbau" (News for Machine Builders")* 5/82. FNT *English title of the paper: "Machinery News"
An instable turbocompressor state in which pumped medium flows from the compression or outlet side back to the suction side in surges or peridically is called surge. Surge occurs when the end pressure is too high and/or the throughput too low. In a characteristic field or graph for a compressor which is defined by end pressure and throughput or by coordinates derived therefrom, it is possible, to unequivocally define a line which separates the stable from the instable zone. This line or curve is called the surge limit. Controlling the surge limit of the compressor is necessary to prevent the compressor's working point from reaching the surge limit, thereby causing surges. Towards this end a blow-off line is established in the characteristic graph at a safety distance from the surge limit. If the working point crosses the blow-off line, a relief valve branched off the compressor outlet is opened more or less to blow off pumped or compressed medium or reorient it to the suction side, thereby lowering the end pressure or increasing the throughput.
The surge limit curve and, hence, the blow-off curve are fixed in the characteristic graph unequivocally, unchangeably, and independently of the momentary operating state when the adiabatic head .DELTA.h.sub.ad and the volume of the suction flow V are used as characteristic graph or field coordinates. From the continuously measured compressor operating variables, in particular suction and end pressure, and from the pressure difference at a throttling point on the suction side, these coordinates can be computed by the formulas: ##EQU1## in which P.sub.1 is the suction pressure, P.sub.2 the end pressure .DELTA.P the pressure drop at a throttling point on the suction side and T.sub.1 the temperature on the suction side, these values being present as constantly monitored measured values. R.sub.1 is the gas constant and .kappa. (kappa) is the isentropic index of the respectively pumped gas, while K is a constant depending upon the geometry of the throttling point in the compressor intake. The letter z represents a constant factor (real gas factor).
In the characteristic field defined by .DELTA.h.sub.ad and V, the location of the surge limit and, hence, also of the blow-off ##EQU2## line, is independent of chnges of the parameters contained in the formulas (1) and (2). However, computing these characteristic field coordinates from the measured pressures and the temperatures is possible only if R, .kappa. (kappa) and K are known. At a given, unchangeable compressor geometry and at unchangeable pumped gas composition, these variables R, .kappa., and K can be measured once and then treated as constants. But a change in the pumped gas composition can result in a change of the associated values for R and/or .kappa.. The changes are not directly measurable, however. In such a case, sticking by the previous values for R and .kappa. would lead to a wrong computation of the characteristic field coordinates so that the surge limit curve would also be incorrect in a characteristic field so computed. The situation is similar if the effective compressor geometry is altered, e.g. by dirt.
If the surge limit control is based on such an incorrect surge limit curve and hence, an incorrect blow-off line in the characteristic field, the consequence is either that surging is not prevented with certainty or that opening the relief valve is already triggered at too great a safety distance from the real surge limit, which can lead to undesirably high power losses of the compressor.